The present invention relates to energy generation systems and, more particularly, to a natural convection boiling water reactor where the reactor can be a fission reactor. Specifically, the present invention applies to boiling water reactors which use a chimney to augment coolant circulation and which use free-surface steam separation to extract the steam phase used to deliver energy from the recirculating water phase. A major objective of the present invention is to provide for enhanced energy transfer by reducing carryunder in the fluid circulating in a reactor vessel.
In a boiling water reactor, heat generated by a radioactive core can be used to boil water to produce steam, which in turn can be used to drive a turbine to generate electricity. Natural convection boiling water reactors limit complexity by dispensing with the need for pumping water within the reactor vessel. The nuclear core which generates heat is immersed in water within the reactor vessel. Water circulated up through the core and a chimney above the core is at least partially converted to steam which forms a relatively low pressure head above the core. Water recirculates down a downcomer annulus between the reactor vessel and the chimney plus core assembly. The water in the downcomer is denser than the steam plus water mixture in the core and chimney region. The difference in density forces circulation up through the core and chimney and down through the downcomer.
The chimney directs the steam water mixture vertically from the core. This vertical direction is best effected where the chimney includes multiple vertical sections, each of which serves as a chimney for the portion of the core directly below it. Confining the steam path in this way helps maintain a head of steam above the core, facilitating water circulation.
The steam emerging from the chimney top rises through the water in the reactor vessel and exits through a steam nozzle at the vessel top. Typically, a flat annular array of dryers is disposed near the vessel top to trap any water being carried by the steam and return trapped water to the recirculation fluid. Otherwise, water carried by the steam would limit the efficiency with which the steam can drive a turbine or other energy conversion device. Since there is a net loss of water plus steam from the vessel through the exit port, means are provided to replenish the water in the vessel. This is normally accomplished by returning condensation from the turbine using a fluid handling system, including a feed pump which pumps water through a feedwater sparger which distributes subcooled return water around the downcomer.
"Carryunder", which refers to steam carried in the flow of water recirculating within the vessel and through the core, adversely affects the performance of a natural convection boiling water reactor. Carryunder comprises steam bubbles which have a high thermal energy per unit mass so that they can impair the subcooling provided through the feedwater sparger. The result is a higher water temperature at the core entrance and more rapid boiling of the recirculation fluid as it flows up through the core. The more rapid boiling enlarges the steam voids within the core. The larger voids result in higher irreversible pressure drops through the fuel bundle than would be the case with smaller voids. This effect is amplified, since the larger voids tend to choke recirculation flow, despite a higher driving head. These irreversible head losses can be compensated in the design stage by providing greater chimney height, but this results in a bigger vessel and significantly greater reactor costs.
In addition, the larger voids adversely affect core stability, as the stability-decay ratio is dependent on the proportion of two-phase pressure drop to single-phase pressure drop. This lower stability must be addressed by limiting the power production level below what might otherwise be obtainable. Furthermore, the larger voids create a negative reactivity, requiring the control rods to be withdrawn farther from the core. This reduces the opportunity to achieve long fuel burnups for a given initial core enrichment.
Carryunder results from the inadequate separation of steam and water. Given sufficient time, the different densities of steam versus water would allow adequate separation. In practice, steam is swept along with the radially outward and then downward water flow too rapidly for complete separation. The time available for water and steam to separate can be increased either by reducing the recirculation rate or by increasing the volume available for steam/water separation. However, water flow impacts core void size and thus the efficiency with which neutrons generate heat. As an alternative, the reactor vessel can be made larger to accommodate more volumous recirculation paths within the vessel. However, enlarging the vessel not only increases the cost of the vessel, but also requires geometrically larger versions of the multiple containment systems provided for a reactor vessel. Large containment systems require more materials, more maintenance, and greater potential exposure of personnel to nuclear radiation or contaminants.
Significant reductions in carryunder have been effected in a reactor system with a height-staggered chimney in conjunction with an elevation-staggered dryer system, as disclosed in U.S. patent application No. 325,839, filed Mar. 20, 1989. The more central chimney sections are taller than the more peripheral chimney sections. The staggered chimney consumes less vessel volume due to the stagger and the volume saved is available to increase the time available for steam to separate from water flow. The disclosed reactor also reduces "carryover", the water trapped in the steam flow to the turbine. This carryover, which can damage the turbine and reduce its efficiency, is addressed using a staggered elevation dryer which increases the volume and hence time available for water to separate from the flow of steam exiting the vessel.
The foregoing advances notwithstanding, further reductions in carryunder are desired to enhance core stability and fuel element lifetimes. What is needed is a natural convection boiling water reactor system which reduces carryunder without requiring a larger reactor vessel and without reducing the volume flow of steam from the vessel or water through the core. In addition, the increased efficiency provided by such a reactor system should be achieved without substantial costs in terms of size, complexity or safety.